Editorial: Evolving roles of piRNAs in solid tumors

According to Global Cancer Statistics 2020, an estimated 19.3 million new cancer cases and almost 10 million cancer deaths occurred in 2020. Solid tumors represent approximately 90% of adult human cancers, hence they warrant significant attention from the research fraternity to improve upon the existing platforms of treatment and management of the malignancy. Only by a better understanding of the biology associated with cancer development and progression can we identify clinically relevant novel molecular targets that can not only improve upon the risk stratification of the patients, but also assist in overall disease management. PIWI-interacting RNA (piRNA) is a class of small non-coding RNA (26-31nt) that interacts with PIWI proteins to form the piRNA silencing complex (piRISC). PIWI is a subfamily of Argonaute, and piRNA must bind to PIWI to exert its regulatory role, although have been also described PIWI independent functions for piRNAs. Nearly 10 million unique piRNA sequences (2) have now been identified in humans alone that have been recognized to play a wide variety of roles including germline development, maintenance, and protection of the genome integrity by repressing the activity of transposons through post-transcriptional silencing or other epigenetic mechanisms. Emerging data suggests that piRNAs also have strong regulatory roles within the somatic tissues where they regulate gene expression by inducing histone modification and DNA methylation. Owing to their remarkable roles in maintaining cellular homeostasis, it is not surprising that the expression of piRNAs is reported to be frequently deregulated in several cancers. Current studies indicated that piRNAs are significantly abnormally expressed and are involved in the initiation, progression, and metastasis of different solid tumors, which may be the potential diagnostic tools, prognostic markers, and therapeutic targets for cancers. This special issue is a collection of original research and review articles on this topic.